Discrete Pore-Scale Models for Drying Particle Aggregates

Zeit:	5. Juli 2012
Referent*in:	Dr.-Ing. Abdolreza Kharaghani Thermal Process Engineering, Otto-von-Guericke-Universität, Magdeburg
Veranstaltungsort:	Pfaffenwaldring 61, Raum U1.003 (MML), Universität Stuttgart
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Drying plays a substantial role in the production and processing of saturated particulate and porous materials. So far several attempts have been made to understand the drying characteristics of porous materials via modeling on different length scales. At the effective medium scale, continuum models based on volume averaging have been developed, in which transport is described by gradients in spatially averaged quantities and controlled by effective parameters. Modern product engineering, however, requires a new approach to study the interaction between porous structure and drying process. Two simulation tools are presented for convective drying of dense as well as highly porous particle aggregates, both operating at the pore scale.

For dense particle aggregates, pore network models are used to describe the void space by a network of geometrically similar pores, and discrete transport rules are set up to describe drying as a sequence of pore emptying. Thus the evolution of liquid distributions can be simulated. In a one-way coupling scheme (liquid - solid), capillary forces are computed over time from the filling state of pores and applied as loads on each particle in discrete element method. If bond strength between particles is exceeded, individual contacts will break up and particles may reorganize, leading to more or less sever structural damage. The influence of liquid phase distributions during drying and material properties on mechanical response (shrinkage and crack) is presented.

In case of highly porous particle aggregates, instead of approximating the void space by a pore network, a discretization of the full domain of interest - consisting of solid, liquid, and gas - is employed: The phase distributions are described by time-dependent cell volume fractions on a stationary cubic mesh. The solid phase volume fractions are computed from an arbitrary collection of spherical primary particles. The volume of fluid method is used to track the liquid-gas interface over time. Local evaporation rates are computed from a finite difference solution of a vapor diffusion problem in the gas phase, and the liquid-gas interface dynamics is described by volume-conserving mean curvature flow, with an additional equilibrium contact angle condition along the three-phase contact lines. Numerical results illustrate the capillary effects commonly observed in experiments. The evolutions of the liquid distribution over time for different wetting properties of the solid surface as well as binary liquid bridges between solid particles are presented.